Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view, the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field of the RF pulse extends perpendicular to the z-axis, so that the magnetization performs a precession about the z-axis. This motion of the magnetization describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z axis to the transverse plane (flip angle 90°). The RF pulse is radiated toward the body of the patient via a RF coil arrangement of the MR device. The RF coil arrangement typically surrounds the examination volume in which the body of the patient is placed.
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of receiving RF antennas or coils which are arranged and oriented within the examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving RF antennas or coils corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to a MR image by means of Fourier transformation or by other per se known reconstruction techniques.
MR rheology has become known recently as a promising technique for gathering diagnostically useful additional information on tissue properties that are not accessible via conventional MR imaging alone. MR rheology utilizes the fact that the MR signal phase in a MR image of the examined object changes under the influence of mechanical oscillations acting on the examined object. The extent of this change is dependent on the local deflection of the tissue caused by the mechanical oscillations. Information regarding mechanical parameters of the tissue, for example, concerning the viscosity or elasticity, can thus be derived from MR phase images acquired from the examined object while mechanical oscillations are acting on the object. A MR phase image means in this context a MR image reproducing the spatial distribution of the phase of the nuclear magnetization.
The mentioned mechanical parameters accessible via MR rheology, like tissue viscosity or elasticity, can otherwise only be determined invasively by means of biopsy and/or histology. On the other hand, it is known that these parameters directly link to, for example, cirrhotic or cancerous changes in liver, breast or brain tissue. It has been demonstrated that MR rheology is especially useful for diagnosis of liver cirrhosis and to determine the stage of liver cirrhosis. Further, MR rheology has been proven to be useful for the diagnosis of breast cancer. Initial applications for the examination of degenerative brain diseases by means of MR rheology have been reported.
In a typical MR rheology setup provision is made for at least one transducer which generates a reciprocating motion at a given frequency. The transducer excites mechanical oscillations in the tissue of the patient's body. Moreover, provision is made for an appropriate arrangement of RF coils for generating MR images of the anatomical background. Basically, the transducer excites a mechanical wave propagating inside the body tissue, wherein the propagation direction is perpendicular to the body surface to which the transducer is attached. An important pre-requisite is good mechanical coupling of the transducer to the patient's body.
An oscillation applicator useable for MR rheology is for example known from U.S. Pat. No. 6,833,703 B2. This known applicator is designed as a mammography accessory for MR rheology which is capable of generating longitudinal oscillations extending in the longitudinal direction in the mammae of a patient to be examined. The known applicator is integrated into the patient table of the MR device and provides good coupling of the transducer to the body.
One drawback of known designs of oscillation applicators for MR rheology is that positioning of the applicator on the patient's body is not possible for all required imaging positions. A further issue is that transducers based on electro-magnetic drives (such like electric motors or linear electro-magnetic oscillators) interact with the main magnetic field B0. Consequently, such transducers can be positioned within the examination volume of the MR device only in such a manner that the magnetic fields generated by the electro-magnetic drives is oriented perpendicular to the field lines of the main magnetic field B0. This restricts the placement of the oscillation applicator and, consequently, the application of MR rheology for certain body regions.
The paper ‘Effects of gadoxetic acid on liver elasticity measurement by usiing magnetic resonance elastography’ by U. Motosugi et al. in Magn. Res. Im. 30(2011)128-132 mentions the use of a passive driver attached to an elastic belt to deliver vibrations to the patient's chest and liver.
From the foregoing it is readily appreciated that there is a need for an improved oscillation applicator for MR rheology.